This invention relates in general to pseudocolor images and more particularly, to a real-time analytic pseudocolor system and encoder.
Most of the images obtained in various scientific and medical equipment are usually in the form of gray-level images. For example, thermal line scan recorders, laser retina scanners, and multi-format cameras normally produce only gray-level images. Other relatively widely known density images such as scanning electron microscopic images, X-ray transparencies are all gray-level images.
The human visual system can discriminate simultaneously only 15 to 20 gray levels from a complex black and white image. If the same image is presented in full color, the visually distinguishable levels can be increased enormously, up to hundreds or even thousands of different levels [H.-K. Liu and J. W. Goodman, "A New Coherent Optical Pseudocolor Encoder," Nouv. Rev. Optique, t.7, No. 5, 1976, 285].
The basic philosophy of pseudocolor is that the eye can perceive many more colors than gray levels and pseudocolor mapping will effectively extend the range of the observer's eye [C. H. Radewan, "Digital Image Processing with Pseudo-Color," Conf. Proc. Acquisition and Analysis of Pictorial Data, The Modern Science of Imagery, Aug. 19-20, 1974, Soc. Photo-Optical Instrumentation Engrs. (SPIE), pp. 50-56]. However, this range extension is not only used for observing an already available density image. Surface structure, optical interferant pattern, thermal pattern, etc. can be directly input to the pseudocolor encoder through an appropriate TV camera resulting in a real-time pseudocolor output.
True color information in the input domain is lost when pseudocolor is generated in the output domain. However, under some conditions, true color information is not as important as brightness information. For instance, most animals are color blind. They do not perceive true color information. A pseudocolor encoder picks up the input information as an animal's eye and displays it as a human color perception.
Also, the comparison between two pseudocolor maps is easier than that of density maps. In thermal imaging, if temperature is encoded with gray level, it is very difficult to recognize precisely the temperature of a specific gray level. However, a pseudocolor map may overcome this difficulty. It is much easier to pinpoint a specific color than a specific gray level. The pseudocolor encoder has two primary merits: (1) better discrimination, and (2) better recognition.
Pseudocolor encoding is commonly achieved through two methods: a sophisticated digital method and a relatively simpler optical method. The digital computer technique is a logical choice if the images are already digitized.
In the digital computer technique, the input image is first sampled resulting in the image consisting of finite image elements; for instance, 512.times.512 elements. The input analog gray level signal at each image element is digitized to certain quantized gray levels; for instance, 64 gray levels. The quantized gray level of the image element is then stored in the memory. The computer has a program, which is usually a lookup table, to assign every quantized gray level with a specific color. The programmer may assign the color first based on perceptual terms which relate to attributes of sensations of light and color. The selected perceptual colors are encoded (color codes) with specific data as outputs from the lookup table in the computer. A color/graphics adapter card is implemented in the computer to interface with a video monitor. The adapter card inputs display information from the computer through data bus and address bus, and outputs color video and sync signals to drive the color monitor. The digital computer technique requires a computer for processing and storing the image data, in addition to a color/graphics adapter card. Although it is very flexible in programming, it is relatively expensive.
In contrast, optical methods are simpler. In the above-referenced article "A New Coherent . . . . ", Liu, et al. described a coherent method for implementing pseudocolor encoding by half-tone screen. The principle of the optical half-tone screen method is as follows: the positive and the negative images of the gray-level input are encoded with two primary colors; for example, red and blue. Then the product of the positive image and the negative image is encoded with the third primary color, in this case green. The superposition of these three color-encoded images results in a pseudocolor image of the gray-level input. The optical system was improved by Yu, as described in U.S. Pat. No. 4,623,245 ("Yu"), in which a white-light source was used replacing lasers. Yu also described in U.S. Pat. No. 4,623,245, "System of White-Light Density Pseudocolor Encoding with Three Primary Colors," that the pseudocolor-encoded image formed at the output plane can be received by an additional color TV camera and then depicted on a color TV monitor.
The principle of the optical method is that three optical masks for three primary colors are generated independently from the same density image following three simple analytic transform functions. The superposition of the three color images results in a pseudocolor output. The main difference between the digital computer technique and the optical technique is that the optical technique utilizes continuous analytic transform functions for direct generation of primary color signals instead of quantized discrete gray-levels such as entries in a look up table. The disadvantage of the optical technique is that the generation of optical masks cannot be performed in real-time. Some optical methods employing liquid crystal televisions are able to perform pseudocoloring in real-time; however, the color and contrast are severely limited by the physical properties of the liquid crystal molecules (F. T. S. Yu, S. Jutamulia, T. W. Lin, X. L. Huang, "Real-Time Pseudocolor-Encoding Using Low-Cost Liquid Crystal Television," Opt. Laser Tech. 19, 1987, 45). In another method proposed by Yu (F. T. S. Yu, S. Jutamulia, E. Tam, "Gray Level Pseudocolor Encoding Using a Liquid Crystal Television," J. Opt., (Paris), 19, 1988, 129), primary color images generated by liquid crystal televisions are optically added. To display the pseudocolor-encoded pattern on a TV monitor, an additional color TV camera is required to receive the color images generated on the liquid crystal televisions, since a conventional TV monitor (non-liquid crystal) exhibits no birefringence effect for producing color.